Long-term changes in neuronal function have been reported in connection with extended stays in a microgravity environment. Although some correlations may be made with metabolic effects of extended stays in space, these mechanisms cannot readily be used to explain specific changes in nervous system function. A systematic examination of neuronal dynamics at the cellular and molecular level is needed to understand how extended exposure to microgravity can result in compromised neuronal function. The experiments in this application will provide a basis for defining the molecular basis for changes in neuronal plasticity, connectivity, and function. Particular attention will be paid to identifying regulatory pathways which may be used to ameliorate or reduce potentially deleterious changes in the nervous system associated with extended stages in space. Two specific aims will be addressed: 1) To determine the effects of space flight on the dynamics, organization, and composition of the neuronal cytoskeleton. Long-term changes in neuronal function have been reported following extended stays in a space environment. Cytoskeletal elements form the structural basis for neuronal architecture and dynamics. Since changes in the composition and organization of the neuronal architecture and regeneration, the plasticity of a neuronal population is closely linked to dynamics of the cytoskeleton. Specific properties appear to be locally modulated by the microenvironment of the axon and interactions with target cells, so the conditions of space flight may adversely affect neuronal connectivity and plasticity through several mechanisms, including changes in patterns of synaptic activity, and alterations in the axonal microenvironment associated with microgravity or stress. Experiments in this aim are designed to characterize the effects of space flight on the axonal cytoskeleton and identify underlying mechanisms. 2). To evaluate molecular mechanisms of vesicle trafficking in the presynaptic terminal important for neuronal plasticity and synaptic transmission. Sustained release of neurotransmitter requires precise coordination of vesicle movements, targeting of organelles, sorting of membrane proteins, turnaround of fast axonal transport, and recycling of synaptic vesicle constituents. While considerable progress has been made toward understanding some of the associated molecular mechanisms such as fast axonal transport, relatively little is known about the molecular signals, motors, or sorting machinery associated with vesicle trafficking in the presynaptic terminal. Alterations in vesicle recycling may affect maintenance of synaptic terminals and stability of connections through a loss of trophic interactions or disruption of signalling pathways mediated through axonal transport. Experiments in this aim are designed to define molecular mechanisms that control vesicle trafficking and to identify intermediates that might be affected by the conditions of space flight.